This invention relates to a particle measurement system for determining the characteristics of particles in liquid from the rays of light scattered by the particles, and, more particularly, to an apparatus for measuring particles in liquid that eliminates stray light, which will produce noise during detection, and measures the particles so that the number of particles in the liquid is counted without omission.
The apparatus may be used to monitor dust in liquids which are used in the semiconductor manufacturing process or determine the size and kind of bacteria and biological cells in bioindustry.
Many improvements have been made to enhance the SN ratio during detection in a device in which the sample suspension flows through a transparent tube (flow cell) and light is irradiated against the sample suspension to determine the characteristics of particles contained in the suspension such as size and refractive index by detecting the scattering light from the particles, with most of the improvements relating to efforts to reduce the stray light that will produce noise, or eliminate the effect of the noise.
Unlike the measurement of particles in gases, the measurement of minute particles in liquids has the following problems. More particularly, since there are differences in the refractive index among the sample liquid, the material forming the flow cell through which the sample liquid is passed, and the open air, the scattering light and the reflected light from the boundaries between the three materials will form strong stray light.
To eliminate the strong stray light several conventional methods have been proposed as in, for example, Japanese Patent Laid-Open No. 114260/1979. A first proposed method consists in making the measuring region in the sample suspension very small, keeping the sensitive region as far away as possible from the boundary between the sample suspension and the wall of the flow cell to protect against the effect of stray light from the boundary. However, a disadvantage of this proposed method resides in the fact that since the measuring region is very small, the amount of sample liquid that can be measured per unit time duration is very small and that particles in regions other than the measuring region may not be counted by mistake. A second proposed method forms a flow of liquid around the sample suspension stream that has a refractive index equal to that of the sample suspension to eliminate stray light from the boundary between the sample suspension and the wall of flow cell, a method that uses a so-called sheath flow cell. With this method, since the stream of the sample suspension can be made narrower than the diameter of the irradiated light beam, a failure to count can be prevented. However, disadvantages of this proposed method resides in the fact that the sheath liquid flowing around the sample suspension stream must be a clean liquid containing no particles, and, to prevent the sample suspension stream from dispersing, it is necessary to produce a flow of sheath liquid which moves at higher speed and in larger quantity than the sample suspension. This means that the flow of the sheath liquid determines the upper limit of the sample suspension flow, so that in practice the flow of the sample suspension per unit duration of time is very small.
An apparatus to measure the characteristics of particles in a gas by detecting scattering light is also proposed in Japanese Patent Laid-Open No. 69683/1976 which employs a method of pouring the sample gas containing particles from a nozzle and irradiating light coaxially with the axis of the sample gas flow. Since the refractive index of the sample gas and that of the surrounding gas are almost equal, the irradiated light cannot be contained in the sample gas. To prevent any ommision in particle counting, a light beam larger in diameter than the sample gas stream must be applied and this means that there is an excess quantity of the irradiated light that is not effectively used. Also since the sample gas is poured out into another gas, the surrounding gas needs to be moved in the same steady flow as the sample gas to prevent sample gas flow dispersion.
Unlike the particle measurement of gas, measurement of particles in liquid must detect particles contained in a liquid which has a different refractive index than that of air. Therefore, the reflected light, diffracted light and scattering light from the surface of the flow cell through which the sample liquid is passed combine to form strong array light. To avoid the influence of the stray light, the common techniques that are available so far, as mentioned earlier, include focusing the incident light on a very small region of the sample liquid or pouring the sample liquid in a narrow stream by utilizing the sheath flow cell. These conventional techniques have the drawbacks that the entire cross section of the sample liquid stream is large as compared to the sensitive region, wasting a large proportion of sample suspension flow, and that when the sample suspension stream is made narrower than the diameter of the irradiated light beam to eliminate any omission in particle counting, the portion of the beam that is not applied to the sample suspension is wasted.